The present invention relates to a method for control of a combustion engine and the arrangement of a combustion engine having means for generation of an air/fuel mixture to each cylinder of the engine, an exhaust system which is connected to the engine, an exhaust catalyst and a NOx adsorbent which are provided in the exhaust system thereof. The invention is particularly intended for use in the field of regeneration of a NOx adsorbent which is arranged in connection with a combustion engine. In a particularly preferred embodiment, the invention takes the form of a device for control of a combustion engine including means for generation of an air/fuel mixture to each cylinder of the engine, an exhaust system which is connected to the engine, an exhaust catalyst and a NOx adsorbent which are provided in the exhaust system. The generating mean(s) are adapted to assume: a first operating condition that delivers a comparatively lean exhaust gas mixture to the NOx adsorbent for adsorption of NOx compounds in the exhaust gas mixture; and a second operating condition that delivers a comparatively rich exhaust gas mixture to the NOx adsorbent for desorption of NOx compounds in the exhaust gas mixture.
In the field of vehicles which are operated by combustion engines, there is a general demand for low emissions of harmful substances in the exhaust gases from the engine. These substances are primarily considered to be pollutants and often take the form of nitrogen oxide compounds (NOx), hydrocarbon compounds (HC), and carbon monoxide (CO). Regards today""s petrol engines, the exhaust gases are normally purified by means of an exhaust catalyst, which forms part of the exhaust system and through which the exhaust gases are guided. In what is often referred to as a three-way catalyst, which is previously known, the major part of the above-mentioned harmful compounds are eliminated by catalytic reactions. In order to optimize the function of the catalyst so that it provides an optimal degree of purification for NOx, HC, and CO, the engine is in most operating cases operated by a stoichiometric air/fuel mixture, i.e. a mixture where xcex=1.
Although today""s three-way catalysts normally have a very high degree of purification which strongly reduces the emissions of harmful pollutants into the atmosphere, there are demands today for additional reductions of the emissions of such harmful substances. These demands originate from among other things, increasingly strict legislation in various countries, with associated demands for extremely low emissions of NOx, CO, and HC compounds.
Furthermore, in the field of vehicles, there is a general demand for reducing the fuel consumption of the engine to the highest possible degree. To this end, during the last few years, engines have been developed having new types of combustion chambers in the engine""s cylinders, particularly in order for the engine to be able to be operated by increasingly lean fuel mixtures, i.e. where xcex=1. Such engines are generally termed xe2x80x9clean-burnxe2x80x9d engines. In one type referred to as a DI engine (i.e. a direct-injected Otto cycle engine), the respective combustion chamber in the engine is constructed in such a manner that the supplied fuel can be concentrated to a high degree at the respective spark plug. During continues driving, such engines can be operated by a very lean air/fuel mixture, approximately xcex=4. For this reason, a substantial savings in fuel consumption is obtained using this type of engine.
Due to the fact that a DI engine is normally operated by a very lean air/fuel mixture, a correspondingly lean exhaust gas mixture will flow through the three-way catalyst. This results in the three-way catalyst being unable to reduce the NOx compounds in the exhaust gases (due to the fact that it is constructed for an optimal degree of purification for a stoichiometric mixture). For this reason, a conventional three-way catalyst can be combined with a nitrogen oxide adsorbent (also called NOx, adsorbent, or xe2x80x9cNOx trapxe2x80x9d), which is a per se a known device, for absorption of NOx compounds, e.g. in the exhaust gases from a combustion engine. In this manner, the NOx adsorbent can be installed and utilized as a complement to a conventional three-way catalyst.
A NOx adsorbent can be arranged either as a separate unit upstream of a conventional three-way catalyst, or alternatively as an integral part of the three-way catalyst, i.e. together with the catalytic material of the three-way catalyst. The NOx adsorbent is constructed in such a manner that it takes up (adsorbs) NOx compounds from the exhaust gases if the engine is operated by a lean air/fuel mixture and gives off (desorbs) NOx compounds if the engine is operated by a rich air/fuel mixture during a certain time period. Furthermore, the NOx adsorbent has the property of being able only to adsorb NOx compounds up to a certain limit, i.e. it is eventually xe2x80x9cfilledxe2x80x9d and thus reaches a limit for adsorption. In this situation, the NOx adsorbent must be regenerated, i.e. it must be influenced to desorb, and thus to release the accumulated NOx, compounds. If a conventional three-way catalyst is arranged downstream of the NOx adsorbent, the desorbed NOx compounds can be eliminated by means of the three-way catalyst, provided that the latter has reached its ignition temperature.
According to known designs, the regeneration of the NOx adsorbent can be accomplished by making the exhaust gas mixture through the NOx adsorbent comparatively rich during a certain time period, which in turn can be achieved by means of the engine being operated by a comparatively rich air/fuel mixture for a short time period, e.g. a few seconds. In this manner, the NOx adsorbent is xe2x80x9cemptiedxe2x80x9d so that it thereafter can adsorb NOx compounds during a certain time period which lasts until a new regeneration becomes necessary.
Such regeneration is known to be obtainable by means of a control of the air concentration in the exhaust gas mixture through the NOx adsorbent. A system for such a control is disclosed in the U.S. Pat. No. 5,461,857 which discloses an engine having an exhaust gas filter system that in turn comprises a three-way catalyst, an oxidation catalyst and a NOx adsorbent. According to this known system, an air/fuel mixture is combusted in the engine""s combustion chamber, wherein the exhaust gases from the engine are fed through the three-way catalyst in a known manner. Furthermore, the system contains an air pump for supplying secondary air to the exhaust pipe at a position between the three-way catalyst and the NOx adsorbent. By means of the supply of secondary air, the exhaust gas mixture through the NOx adsorbent can be made lean. In this manner, the reduction of the NOx compounds in the exhaust gases takes place in two steps, i.e. in the three-way catalyst, as well as in the NOx adsorbent. In such a system, the NOx adsorbent can be regenerated by interruption of the supply of the secondary air. This provides a pulse with a comparatively rich exhaust gas mixture to the NOx adsorbent, by means of which it is regenerated.
Although this known system in principle operates satisfactorily, it has, however, certain drawbacks. Primarily, in can be noted that the system requires a number of different constructional details to supply the secondary air (i.e. a secondary air pump, air conduits, valves, and connections for the air conduits to the exhaust system). These components must be assembled in connection with the engine, which causes the system to become unnecessarily expensive as regards the cost for the components, as well as for its assembly in the vehicle in question.
Another drawback relates to the fact that the driver of the vehicle normally perceives this regenerative process disadvantageous to comfort if the NOx adsorbent is regenerated too often. This results because the production of rich exhaust gas pulses can be perceived as xe2x80x9cjerky,xe2x80x9d and consequently, disturbing to the driver.
In view of the above described deficiencies associated with conventionally designed regenerative NOx adsorbent systems, the present invention has been developed. These enhancements and benefits are described in greater detail hereinbelow with respect to several alternative embodiments of the present invention.
The present invention in its several disclosed embodiments alleviates the drawbacks described above with respect to conventionally designed regenerative NOx adsorbent systems and incorporates several additional beneficial features.
One object of the present invention is to provide an improved method for purification of harmful emissions from a combustion engine. In particular, the invention is intended for control of a combustion engine having: means that controls the air/fuel mixture to each cylinder of the engine, an exhaust system which is connected to the engine, and an exhaust catalyst and NOx adsorbent which are both provided in the exhaust system. The invention controls the means to a first operating condition for generating a comparatively lean exhaust gas mixture to the NOx adsorbent for causing adsorption of NOx compounds from said exhaust gas mixture. In a second operating condition, the control means causes the generation of a comparatively rich exhaust gas mixture to said NOx adsorbent for causing desorption of NOx compounds back into the exhaust gas mixture. The invention is characterized in that it comprises detection of whether at least one condition in the vehicle prevails which corresponds to an initiation of a change-over from said first operating condition to said second operating condition, wherein an adjustment of said air/fuel mixture to the engine is carried out. The generation process of a rich exhaust gas mixture is initiated for desorption of NOx compounds from the NOx adsorbent depending on said condition.
Through the utilization of the invention, several advantages are accomplished. Primarily, it can be noted that the invention provides a reduced fuel consumption for the engine in question, which is due to fact that the NOx adsorbent is regenerated at optimal points as regards the fuel consumption. More precisely, the regeneration occurs during such occasions where an increase of moment occurs in the engine anyway. As a result, according to the invention, no additional, separate occasions (which per se would result in an increased fuel consumption) need to be utilized in order to regenerate the NOx adsorbent. Furthermore, by means of the invention, reduced emissions are obtained as compared with previously known systems. Also, a higher degree of comfort is obtained for the vehicle""s passengers, which is due to the fact that the passengers, to a less extent than in known systems, feel impacts and similar disturbances during driving.
According to a preferred embodiment, the invention includes detecting the amount of NOx compounds in the exhaust gases from the engine. This results in that the invention permits a control of a possible sulphur detoxification in the NOx adsorbent, which in turn enables a prolonged life for the NOx adsorbent. Furthermore, it becomes possible to drive with different fuel qualities for the engine, e.g. fuel which contains a certain amount of sulphur, which is due to the fact that the invention provides a control for possible sulphur detoxification. In this manner, the invention permits the engine system to be flexible as regards different fuel qualities (e.g. fuels having different NOx levels).